The coefficient of friction between the material of the metallic traction sheaves used and the hoisting ropes is often insufficient as such to maintain an adequate grip between the traction sheave and the hoisting ropes in ordinary conditions of elevator operation. The friction and the forces transmitted by the ropes are increased by shaping the rope grooves of the traction sheave, or e.g. by providing the rope grooves with a coating that increases the coefficient of friction. In elevators provided with coated traction sheaves, it is possible in exceptional conditions, e.g. in the event of a fire, that the coating on the surface of the traction sheave is destroyed by being burned or melted off. In such a situation, the coefficient of friction between the traction sheave and the hoisting ropes becomes insufficient and elevator movements can not be controlled. The loss of an adequate grip between the traction sheave and the hoisting ropes is a problematic and dangerous situation in respect of elevator operation and safety. When the coating is damaged, the system's ability to maintain the functionality of the elevator is impaired and at the same time the reliability of the elevator may change as a result of reduced grip especially in a situation where a large imbalance prevails between the elevator car and the counterweight. Such a situation occurs when the elevator has no load or is heavily loaded.
One solution to the problem of reduced grip resulting from loss of the coating is disclosed in specification U.S. Pat. No. 4,465,161. The solution proposed here is to provide the traction sheave with toothings under the coating in order to achieve a better grip between the traction sheave and a steel wire rope after the coating has been destroyed. A problem with such toothings, which are implemented by a known technique, is that the hoisting rope comes into contact with the traction sheave when the coating disappears, and this may damage the hoisting rope and its structure. Especially a heavy contact between the toothing and the hoisting rope may damage the rope, which may even result in the rope being broken. This is a definite safety risk. The reliability of the elevator also changes as a result of the hoisting rope being damaged, which leads to a hazard especially in a situation where the elevator is heavily loaded. The loss of the coating in a traction sheave implemented by prior-art techniques also has the consequence that, after the toothing and the steel wire rope have come into contact, it is often necessary to replace both the traction sheave and the hoisting rope as both have been damaged. This causes considerable additional costs.